The need to eliminate lead-based octane enhancers in gasoline has provided an incentive for the development of processes to produce high octane gasolines blended with lower aliphatic octane boosters. Supplementary fuels are being examined by the petroleum refining industry. Lower molecular weight alcohols and ethers, such as isopropyl alcohol (IPA) and diisopropyl ether (DIPE), are in the boiling range of gasoline fuels and are known to have a high blending octane number. They are also useful as octane enhancers. In addition, by-product propylene from which IPA and DIPE can be made is usually available in a fuels refinery, typically as a C.sub.3 aliphatic stream which is rich in both propylene and propane.
The preparation of DIPE from propylene chemically proceeds by two sequential reactions where propylene is first hydrated to IPA (1) followed by reaction of the alcohol with the olefin (2) or bimolecular reaction of the alcohol (3) (Williamson synthesis) according to the equations, ##STR1##
When DIPE is produced via reaction (3), twice as much IPA is required than when DIPE is produced via reaction (2). Since hydration reactions, for example reaction (1), are generally more difficult to perform than etherification reactions, the production rate of the alcohol limits the overall sequence and it is desirable to limit the formation of DIPE from reaction (3) and increase the formation of DIPE from reaction (2). Side reactions that can occur in this process are the reaction of propylene with itself to make C.sub.6 olefins and the reaction of C.sub.6 olefins with propylene to make C.sub.9 olefins. These reactions are considered undesirable since they result in low value polygasoline with low octane and no oxygen content.
The synthetic production of IPA and DIPE is well known. Among the earliest processes for the production of IPA and DIPE were the so-called "indirect hydration processes". In the indirect hydration process, a selected olefin feed is absorbed in a concentrated sulfuric acid stream to form an extract containing the corresponding alkyl ester of the sulfuric acid. Thereafter, water is admixed with the ester-containing extract to hydrolyze the ester and to form the desired alcohol and ether which are then recovered, generally by stripping with steam or some other heating fluid. A diluted sulfuric acid stream is thereby produced. This acid stream is then generally treated to concentrate the sulfuric acid stream for recycle to the absorption stage.
In the indirect hydration process, the use of sulfuric acid as a catalyst presents certain problems. First, severe corrosion of process equipment can occur. Second, separating the produced ether from the sulfuric acid can be difficult. Third, a substantial quantity of waste sulfuric acid is produced in the concentration of the catalyst for recovery. Because of these problems, it has been found that the process of synthesizing DIPE by using concentrated sulfuric acid is not commercially desirable. Clearly, there was a need for a more direct manner of bringing about the hydration reaction.
This need was addressed by so-called "direct hydration processes" using solid catalysts. In the direct hydration process, an olefinic hydrocarbon such as propylene is reacted directly with water over a solid hydration catalyst to produce an intermediate IPA stream from which the product DIPE can be formed. Development work using direct hydration focuses on the use of solid catalysts such as active charcoal, clays, resins and zeolites. Examples of olefin hydration processes which employ zeolite catalysts as the hydration catalyst can be found in U.S. Pat. Nos. 4,214,107, 4,499,313, 4,857,664 and 4,906,787.
The use of zeolites as hydration catalysts has the disadvantage of the zeolites being expensive in comparison to other catalysts, for example, ion exchange resin catalysts. Also, in comparison to ion exchange resin catalysts, zeolites do not operate as well at the relatively low hydration and etherification temperatures where the equilibrium conversion is at its highest. Therefore, zeolites have a lower conversion per pass since the zeolites are operated at a higher temperature. In the etherification step, when no water is present zeolites have a strong tendency to form DIPE from reaction (3) instead of reaction (2). They also have a strong tendency to produce substantial amounts of undesirable polygasoline from the reaction of propylene with itself.
The use of ion exchange resins in the production of tertiary alkyl ether is well known in the art. G.B. 1,176,620 (issued to Shell) discloses reacting an olefin with an alcohol in the presence of a cation exchange resin containing an SO.sub.3 H group to form a tertiary alkyl ether. The patent teaches that the preferred cation exchange resin be a sulfonated styrene/divinylbenzene co-polymer. Although sulfonated cation exchange resins have enjoyed considerable success as an etherification catalyst, these resins are susceptible to thermal degradation, by the reaction sequence shown below: ##STR2##
U.S. Pat. No. 4,182,914 (issued to Imaizumi) describes a two-stage process for producing DIPE using a strongly acidic ion exchange resin. Two-stage DIPE processes first make the IPA in one reactor and then react the IPA with propylene in another reactor to form DIPE. Accordingly, in a two-stage DIPE process, two reactors are required. In the Imaizumi process, propylene and isopropyl alcohol are fed to a DIPE reactor. A portion of the effluent exiting the DIPE reactor is recycled to the DIPE reactor. The remainder of the DIPE reactor effluent is passed to a neutralization zone to neutralize small amounts of SO.sub.3 which is produced by thermal degradation the DIPE reactor. The neutralizing zone contains a water-insoluble, solid, particulate, acid-neutralizing agent such as magnesium oxides and aluminum oxide. The effluent from the neutralization zone is fed through a series of clean-up steps such as flashing to remove light ends and water wash to remove isopropyl alcohol.
The Imaizumi process recycles DIPE reactor effluent back to the DIPE reactor without treating the reactor effluent to remove SO.sub.3. This is acceptable if SO.sub.3 levels are low, i.e., when the reaction temperature is low, and when water is only present in low levels.
Two-stage DIPE processes, such as the one described in the Imaizumi patent, can be prohibitively expensive because of the capital cost associated with the two reactors. In many instances, a single stage DIPE process is more cost effective. Single stage DIPE processes can be more affordable than two-stage DIPE processes, but the single stage reactor must generally be operated at a higher temperature than the two-reactor system because water is present. As a result, the amount of SO.sub.3 lost by the resin in the DIPE reactor is considerably more than in the etherification section of the two-stage DIPE process. In some circumstances, the amount of SO.sub.3 in the DIPE reactor effluent can be up to 10 to 100 times as high in comparison to the two-stage process. When this high level of SO.sub.3 is recycled back to the single stage DIPE reactor, the concentration of SO.sub.3 in the DIPE reaction zone will increase thereby substantially accelerating the deactivation of the ion exchange resin. The reason the deactivation rate increases in the presence of the SO.sub.3 is that the desulfonation rate is acid-catalyzed. Generally, the rate of desulfonation of the ion exchange catalyst increases 2.5-3.5 for every 1N increase in acid concentration.
There is a need for an economic single stage DIPE process which successfully handles the SO.sub.3 catalyst deactivation problem. It is an objective of the present invention to address this need.